Recently, a noncontact tracing control system using a noncontact distance detector as a tracer head has been developed, and, for example, an optical distance detector is used for this noncontact distance detector, and a model is traced by using this detector to detect the distance to the model surface. This system eliminates fears of damage to the model, and therefore, a soft material can be used for a model, which will enable a wider applicability thereof for machining and digitizing.
The conventional noncontact tracing control system, however, has a problem in that the tracing accuracy is lowered at portions of a model where an inclination thereof is large. Namely, at this portion, an optical measuring axis of a distance detector is not at a right angle to the model surface, and accordingly, the spot of light on the model surface is distorted into the shape of an ellipse, and thus the resolution of the distance detector is lowered and the tracing accuracy becomes low. Particularly, for a trigonometrical distance detector, a measurement sometimes becomes impossible because the optical measuring axis interferes with the model surface due to the angle thereof.
To solve this problem, the applicant filed a Patent Application No. 1-194500, entitled "NONCONTACT TRACING CONTROL SYSTEM", on Jul. 27, 1989. In Patent Application No. 1-194500, tracing accuracy is improved by acquiring a normal vector of a model surface based on measured values of a sampling taken at this time and at the previous time by two noncontact distance detectors of a tracer head, and by controlling the rotation of the tracer head in the direction of the projection of this normal vector when projected on a predetermined plane, to thereby carry out a distance measuring with a high accuracy.
Nevertheless, the optical measuring axis of the noncontact distance detector cannot be made to perfectly conform to the normal direction, depending on an inclination of the model surface, and thus in some cases, the measuring accuracy cannot be improved as much as expected.